I’ve been writing a lot about the broken sewage system in Tijuana causing spills into San Diego. Part of the concern, San Diego officials told me, is that Mexico lacks a system to monitor whether businesses are dumping toxic waste into the sewer system.

That’s part of the reason why it’s risky to reuse any of that river water because, if we don’t know what’s in the water, we can’t be sure how to best treat it.